Historically, a large proportion of Britain's rainfall has soaked into the earth, thus reducing the likelihood of flooding and replenishing groundwater supplies (long term storage which feeds well and springs and acts as a balance between winter and summer water levels). As we continue to pave over vast tracts for housing, highways etc. this water enters the drainage system instead, which has to be designed to take the expected influx. It will not, of course, be designed to accommodate the additional influx from new development after the drainage was designed, so if you build a new housing estate upstream, this increases the likelihood of flooding downstream!

Reservoir storage will do little to ameliorate the falling groundwater levels, but at least then the rainfall will not be "lost", although in order to accommodate major storms, the reserve capacity of any new reservoirs will need to be significant. Due to the quantity of impervious paved area, rainfall now enters the drainage system much more immediately.

As regards flooding, it has been traditional to measure flooding in terms of the level reached by the water, not by the volume of floodwater, which is of course the true measure. So in places like Bewdley, where significant new development alongside the river has taken place both upstream and in the town, the channel width for floodwater has been reduced, the volume of water must then rise to a higher level and the town receives flood levels the Environment Agency believes should only be experienced once every 200 years twice in five months! Note that this is not a harbinger of climatic change and global warming, merely a mis-assessment of the nature of the problem which has been prevalent for 100 years or so.

Anyone still awake?

Jonm was a highways and drainage design engineer in a former existence. The treatment is progressing well.